The lithographic apparatus (lithography machine) is one of important devices for producing large scale integrated circuits. With decreased feature size of integrated circuits and increased resolution of the lithographic apparatus, an effective depth of focus (DoF) for the lithography becomes smaller continuously. The projection lithographic device is a widely used device in the field of lithography. Even if cooperating with a wave-front technology (such as off-axis illumination, and the like), the DoF of the projection lithographic device is maintained on the order of one hundred nanometers, which is far from the fluctuation of the substrate surface caused by vacuum absorption warping, substrate flatness, the thickness of the resist, and the like. In order to maintain the substrate surface within the DoF of the projection system and to ensure the quality of the exposure pattern, some severe requirements have been provided for the focus detection system.
The previous focus detection methods commonly use a slit photometry to detect the focus. However, such a method is limited by the slit image quality and has a severe dependence on the image processing algorithms, and its accuracy is only at a level of a hundred nanometer, which can be only applied to the early lithography with a lower accuracy, such as, a photoelectric measurement method based on a raster and a four-quadrant detector (U.S. Pat. No. 5,191,200), a photoelectric measurement method based on a slit and a four-quadrant detector (U.S. Pat. No. 6,765,647B1), and the like. Subsequently, a focus detection method with a high accuracy based on laser interference (Focusing and leveling based on wafer surface profile detection with interferometry for optical lithography [C] Proc of SPIE, 1994, 2197: 980-989) has been proposed. However, a multiple layer interference of the resist film and fragile environment limit its application for using as a lithographic tool. Recently, the technology of producing a Moire fringe by using a grating having close periods has been introduced to the focus detection. The nanoscale accuracy and good processing adaptability enable the process become a promising alternative solution for focus detection, such as the photoelectric measurement method based on Moire fringe of gratings (CN: 200710171968). However, the Talbot effect due to the self-imaging of grating will reduce the contrast of Moire fringes, and have a negative effect on the debugging. Thus, it is hard to apply the Moire fringe method to the practical focal detection. Currently, the photoelectric measurement method based on gratings is still the most popular method for focusing and leveling. Although the method is of high accuracy and easy to implement, it is susceptible to the changes in light intensity which may be caused by the fluctuations of the source or the reflectance of the substrate surface, thereby decreasing the detection accuracy.